FIG. 1 schematically illustrates four-wire telephone operation. One pair of wires carrier a signal from the microphone MIC of a first handset to the receiver RCVR of a second handset. (Intermediate electronics are, of course, not shown.) Another pair carries a corresponding signal, but in the reverse direction, from the second to the first handset.
Two-wire operation is perhaps more common than four-wire operation. One reason is that, historically, as telephone networks were installed, the less expensive option of installing two, rather than four, wires was taken.
In two-wire operation, a pair of wires carries both signals, which travel in two directions, as shown in FIG. 2. Hybrid circuits, also called duplexers or simply "hybrids," separate the signals, as indicated. In a sense, the hybrids convert four-wire operation into two-wire operation, and vice-versa.
FIG. 2 illustrates two-wire transmission carrying a local call, which does not involve transmission between Central Offices (CO's). However, two-wire service is not required in local calls; in some types of local calls, four-wire transmission is employed. In addition, if the call does require CO-to-CO transmission, then four-wire transmission is generally used.
FIG. 3 illustrates the design of a common hybrid. Z.sub.L represents the impedance of the two-wire line feeding the hybrid. A figure of merit for the hybrid is transhybrid loss (THL), which is given by the expression .vertline.Z.sub.h +Z.sub.L .vertline./.vertline.Z.sub.h -Z.sub.L .vertline., wherein the Z's represent the impedances shown in the Figure.
Transhybrid loss is a measurement of the coupling from the hybrid's INPUT PORT with its OUTPUT PORT, which are so-labeled in FIG. 3. To completely isolate the INPUT PORT from the OUTPUT PORT, THL should be infinite.
However, THL is deliberately made non-infinite, in order to introduce some coupling, or feedback. Some feedback is desirable, primarily to allow a user to hear the user's own voice in the receiver, or earpiece, of the handset. This feedback is termed "sidetone." Without sidetone, the telephone would sound dead.
Even without deliberately attempting to create sidetone, which requires non-infinite THL, nevertheless, THL will, in practice, attain non-infinite values in almost all situations. The main reason is that Z.sub.L in the expression above is almost always a complex quantity (as the term "complex" is used in electrical engineering), and is actually a distributed impedance, whereas the Z.sub.h 's within the hybrid are lumped impedances. An exact match between the lumped Z.sub.h and the distributed Z.sub.L, which is required to attain infinite THL, is highly unlikely. Further, even if a match is attained, it will exist only at a single frequency. Therefore, the occurrence of sidetone is, for practical purposes, inevitable.
Moreover, when sidetone is deliberately introduced, by choosing a non-infinite THL, the sidetone will not remain constant, because of the variance in THL described immediately above: the sidetone depends on THL, and THL varies as Z.sub.L varies.
Therefore, prior-art hybrids produce sidetones. The sidetones have a loudness level which depends on the impedance of the line, which is different at different locations. Moreover, for a single telephone at a single location, sidetone can vary as conditions vary. For example, picking up an extension telephone set will add a parallel impedance to Z which changes the original Z.sub.h.
Sidetone varies for another reason. If a volume control is used to change the volume of the received signal, the control also changes the sidetone level. The reason is that, in practice, the received signal inevitably contains sidetone, because the transmitted signal, containing the user's voice, is reflected by the telephone transmission system. Therefore changing the level of the received signal will also change the overall sidetone level.
In addition, even if there were no reflections, and even if the absolute level of the sidetone were held fixed, changing the volume of the received signal would change the relative volume of the sidetone, which may be undesirable. For example, if the received signal were increased in volume, the ratio of received signal to the (fixed) sidetone would increase.
It may be possible, by using elaborate and expensive filtering schemes, to isolate sidetone from the received signal. However, commercially available telephone sets do not generally use such schemes, primarily to avoid excess cost and complexity.